Download Physics 200 Class #1 Outline

Physics 200 Class #8 Notes
October 3, 2005
Reading Assignment for Wednesday
Text: Chapter 5 pp. 106-118
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Finish interference
Review of Electric Forces (Charge)
Introduction to Electric and Magnetic Fields
Finishing up interference: thin film interference
Thin Film Interference
You get a reflection off the front surface (ray 1) and off the back surface (ray 2) of the thin film.
For normal incidence (incident angle equals zero degrees): The geometric path difference is 2d
Note that the wavelength in the material is different than the wavelength in air. ' =/n
So if the total path difference is equal to 1 wavelength ('), you have both ray 1 and ray 2 in phase
and you get constructive interference. A bright area.
Mathematically, you get constructive interference when:
2d = '
Really, this works for 2 wavelengths, 3, 4, etc., so using "m" to stand for any integer.
2d = m' for constructive interference.
Skip ahead to Electricity and Magnetism if you don't want any more details on
thin film interference.
Now, since we are dealing with materials with different indices of refraction, we have to remember
that when light reflects, there is sometimes a 180 degree phase change. If the material it bounces
off of has a higher index of refraction than the material it has been traveling in, then you get that
phase change, or "phase flip" as it may be called.
In air (n=1.003), light bouncing off of an interface with a piece of glass (n=1.5) will have the phase
flip. In oil (n=1.5 for example), light bouncing off of a blob of water (n=1.33) WILL NOT.
You see? Now apply this to mica (n=1.6) and air (n = 1).
Phy 200 Fall 2005 Class_8
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So, in the air/mica/air example in the diagram, the light off the front of the mica (air to mica
interface) will flip. The light that travels through the mica and bounces off the back surface (mica
to air interface) will not. So with one phase flip:
2d = m' for destructive interference.
If you had diamond stuck on the right side of the mica, so you had a mica-to-diamond interface
(ndiamond = 2.42) the bounce off that second interface would also have a phase flip. Then both
reflected rays having the same phase shift and you'd have
constructive interference with 2d = m'
That may be alot to remember, but remember that you have light bouncing off of both surfaces of a
thin film and that causes interference. And since it's wavelength dependent, you get different
colors interfering differently.
Now the pretty part Since wavelength is in that equation, you get different interference for different colors.
Look at a slick of oil on a wet road. This is just thin film interference.
from http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/phopic/gaspot.jpg
Now - electricity and magnetism
Contributions of Ben Franklin (1706-1790) and Charles Coulomb (1736-1806)
Source of Electric phenomena: Electric Charge (positive and negative)
(We now know that Electric charge is “quantized” that is it comes in integer multiples of a
fundamental unit-the charge on the electron.)
Phy 200 Fall 2005 Class_8
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Like Charges Repel
Unlike Charges Attract
Demonstrations:
Charging by Friction - fur (becomes +) and vinyl (becomes -)
Ben Franklin first categorized charge as + and -.
The negative one (electrons) happens to be the one that does most of the moving, we later
found out.
Charging by Scotch Tape
Induced charges
Why does the balloon stick to the wall?
Insulators: cork, glass, rubber
Conductors: metals
Conservation of Electric Charge: Electric Charge cannot be created or destroyed it can only be
transferred from one object to another. The total amount of charge in the universe is constant.
Force: Push or Pull
And (in this case) grows weaker with distance - watch how much the fur moves.
And depends on how much stuff you have:
mass (for the gravitational force) or charge (for the electric force)
The Electric Force (Coulomb’s Law)
qq
FE  k 1 22
d
q1 and q2 are in Coulombs (a rather large unit; The charge on the electron is 1.6 x 10-19 C.)
d is in meters
FE is in newtons
k is a universal constant and must carry units: k = 9 x 109 N m2/C2
Comparing the Electric and Gravitational Forces: The forces between an electron and a proton in
the hydrogen atom:
31
2
M eM p
kg)(1.7 x 10 27 kg)
11 N  m (9.1 x 10
FG  G

6
.
7
x
10
 3.7 x 10  47 N
r2
kg 2
(5.3 x 10 11 m) 2
FE  k
Qe Q p
r2
 9 x 10 9
N  m 2 (1.6 x 10 19 C )(1.6 x 10 19 C )
 8.2 x 10 8 N
2
11
2
C
(5.3 x 10 m)
FE is 10 39 times stronger
Note the similarity to the gravitational force. This is in general difficult to handle mathematically
and is also “action at a distance”. Enter Faraday with his “Electric Field”. The field concept is an
attempt to get around the “action at a distance” force.
q
force
E
 k 22
with a direction given by the force on a  charge
q1
r
Then: F  q1 E
Phy 200 Fall 2005 Class_8
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The units for E must be newtons/coulomb
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Demonstrations
It is difficult to visualize the static electric field, but we can easily visualize a magnetic field and
we will do this shortly. We can demonstrate the energy contained in the electric field by using the
Van deGraaff Generator or by charging a “capacitor”.
Representing the Field: We represent the field by drawing arrows giving the direction and
strength (length) at each point in space surrounding a charge. It is not reasonable to draw too many
arrows since the picture would soon become too complicated. It is conventional to simply draw
one long arrow away from a positive charge. Keep in mind that the field gets stronger as we
approach the charge.
E points away from positive charges and towards negative charges.
Examples: See text
Magnetic Fields
It is easy to visualize the magnetic field lines around a bar magnet using iron filings.
Now the question is: How do we find the force on a charge?
Experiments show that the force on a stationary charge is zero and the force on a moving charge is:
F  qvb if v is perpendicu lar to B plus a direction given by the right hand rule!
F  qvB sin( angle between v and B)
Phy 200 Fall 2005 Class_8
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